PULSED ULTRAVIOLET-LASER PHOTOLYSIS OF SUBSTITUTED PHENYL AZOSULFONATES - WAVELENGTH DEPENDENT EFFECTS

Aqueous solutions of several substituted phenyl azosulphonates have been irradiated by excimer laser pulses at 308 nm, and by dye laser pulses in the wavelength range 370-390 nm. For the methoxyphenyl azosulphonates, the pathway of photochemical decomposition is found to be strongly wavelength dependent. On a first route, the azosulphonate functional group (-N=N-SO3-) is split off in a one-step reaction, to produce the corresponding methoxyphenol. The second photolysis pathway proceeds via the methoxydiazonium ion as an intermediate, to yield the same final product. The wavelength dependence is sensitive to the substitution geometry on the aromatic ring, i.e., 2- and 4-methoxyphenyl azosulphonates exhibit a complementary behaviour with respect to one- and two-step decomposition. The results are interpreted in terms of semiempirical calculations, in which the changes in phenyl-nitrogen and nitrogen-sulfur bond orders are compared for n --> pi* and pi --> pi* excitations of the parent molecule.